1. Field of the Invention
This invention relates to an improved method for authorizing a radio transmission of emergency commands from a head of train unit to its associated rear unit, and more particularly to a method which requires only one operator to execute the authorization procedure.
2. Description of the Prior Art
End of train (EOT) devices are in common use on several major railroads. These devices are typically mounted on the trailing coupler of the last car in the train and are equipped with pressure monitoring and telemetering circuitry. A hose is connected between the train's brake pipe and the EOT device so that the air pressure of the brake pipe at the end of the train can be monitored. A radio signal transmits the monitored pressure to a head of train (HOT) device mounted on the console in the locomotive cab. The HOT device provides the operator with a display of brake pipe pressure at the end of the train and, in response to a command from the EOT device, signals the operator that an emergency condition exists, such as a sudden loss of air pressure or air pressure falling below a predetermined level. When initially implemented by the railroads, transmissions were one way; from the end of the train to the head of the train only.
More recently, standards have been adopted by the railroad industry which allow two way radio transmissions in HOT/EOT systems. One application for a transmission from the HOT unit to the EOT unit is for the remote control of the emergency application of brakes at the end of the train. In a one way system, emergency application of the brakes starts at the locomotive and progresses along the brake pipe to the end of the train. This process can take a significant time in a long train, and if there is a restriction in the brake pipe, the brakes beyond the restriction may not be actuated. With a two way transmission system, emergency braking can be initiated at the end of the train at the same time as the initiation at the head of the train and the process of brake application can be considerably shortened. As will be appreciated by those skilled in the art, in order for an HOT unit to communicate emergency commands to an associated EOT unit, it is desirable for the HOT unit to be armed or authorized by railroad personnel. This is desirable to prevent one HOT from erroneously or maliciously actuating the emergency brakes in another train. To this end the HOT unit includes a nonvolatile memory in which a unique code identifying an EOT unit can be stored. The HOT also has a row of thumb wheel switches. The HOT is armed to transmit an emergency command to an EOT unit only when an operator at the HOT sets the thumb wheel switches to correspond to the EOT identifying code stored in the nonvolatile memory. The HOT unit retains the last used EOT identification code until rearmed for a different EOT.
To arm the system for a new EOT unit, an operator at the EOT unit pushes a test button which initiates a transmission of the EOT's unique identifier code along with a prescribed special message type identifier and confirmation bit. When the HOT unit receives the message initiated by the EOT push button, it displays an ARM NOW message at the HOT if the stored code differs from the identification code of the new EOT unit. The operator at the HOT must initiate a status update request by manually pushing the Communications Link push button within six seconds of the ARM NOW message display. The EOT unit is programed to respond to the status update request addressed to it if the request is received within six seconds from the time the EOT unit button was pushed. The response has a prescribed special message type identifier and confirmation bit. When the HOT unit receives the response, if the EOT identifier code corresponds to the code set by the thumb wheel switches, ARMED is displayed and the identification code of the new EOT unit is written into the nonvolatile memory in place of the previously stored code. At this point in the procedure the system is armed. This procedure is set forth in the 1989 Association of American Railroads Communications Manual, Part 12-15, pp. 38, 39.
This coordinated action arming procedure uses an operator at the end of the train to insure that the system is armed only by the EOT on the end of that particular train. It depends on operator intervention to associate the HOT unit with the proper EOT unit.
While generally satisfactory, the system has certain limitations. It requires two operators, one located at head of the train and the other located at the end of the train. In many operating situations there is no operator at the end of the train and someone has to be sent to the end of the train in order to arm the system. Not only is the two operator system inefficient when there is no operator at the EOT unit, but it also poses the small but real possibility of operator misidentification.